The course which these streams have followed from
their craters, can often be followed by the eye. The town of Angra is
overlooked by a crateriform hill (Mount Brazil), entirely built of thin
strata of fine-grained, harsh, brown-coloured tuff. The upper beds are seen
to overlap the basaltic streams on which the town stands. This hill is
almost identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.

EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.

In the central part of the island there is a spot, where steam is
constantly issuing in jets from the bottom of a small ravine-like hollow,
which has no exit, and which abuts against a range of trachytic mountains.
The steam is emitted from several irregular fissures: it is scentless, soon
blackens iron, and is of much too high temperature to be endured by the
hand. The manner in which the solid trachyte is changed on the borders of
these orifices is curious: first, the base becomes earthy, with red
freckles evidently due to the oxidation of particles of iron; then it
becomes soft; and lastly, even the crystals of glassy feldspar yield to the
dissolving agent. After the mass is converted into clay, the oxide of iron
seems to be entirely removed from some parts, which are left perfectly
white, whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses are
marbled with two distinct colours. Portions of the white clay, now that
they are dry, cannot be distinguished by the eye from the finest prepared
chalk; and when placed between the teeth they feel equally soft-grained;
the inhabitants use this substance for white-washing their houses. The
cause of the iron being dissolved in one part, and close by being again
deposited, is obscure; but the fact has been observed in several other
places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in
the Italian volcanic islands. Dolomieu says the iron at the Panza Islands
is redeposited in the form of veins (page 86 "Memoire sur les Isles
Ponces"). These authors likewise believe that the steam deposits silica: it
is now experimentally known that vapour of a high temperature is able to
dissolve silica.) In some half-decayed specimens, I found small, globular
aggregations of yellow hyalite, resembling gum-arabic, which no doubt had
been deposited by the steam.

As there is no escape for the rain-water, which trickles down the sides of
the ravine-like hollow, whence the steam issues, it must all percolate
downwards through the fissures at its bottom. Some of the inhabitants
informed me that it was on record that flames (some luminous appearance?)
had originally proceeded from these cracks, and that the flames had been
succeeded by the steam; but I was not able to ascertain how long this was
ago, or anything certain on the subject. When viewing the spot, I imagined
that the injection of a large mass of rock. like the cone of phonolite at
Fernando Noronha, in a semi-fluid state, by arching the surface might have
caused a wedge-shaped hollow with cracks at the bottom, and that the rain-
water percolating to the neighbourhood of the heated mass, would during
many succeeding years be driven back in the form of steam.

TAHITI (OTAHEITE).

I visited only a part of the north-western side of this island, and this
part is entirely composed of volcanic rocks. Near the coast there are
several varieties of basalt, some abounding with large crystals of augite
and tarnished olivine, others compact and earthy,--some slightly vesicular,
and others occasionally amygdaloidal. These rocks are generally much
decomposed, and to my surprise, I found in several sections that it was
impossible to distinguish, even approximately, the line of separation
between the decayed lava and the alternating beds of tuff. Since the
specimens have become dry, it is rather more easy to distinguish the
decomposed igneous rocks from the sedimentary tuffs. This gradation in
character between rocks having such widely different origins, may I think
be explained by the yielding under pressure of the softened sides of the
vesicular cavities, which in many volcanic rocks occupy a large proportion
of their bulk.